In general, in a production step of a semiconductor device, a fine pattern is formed using a photolithography method. Further, in forming the fine pattern, a large number of substrates, which are called transfer masks (photomasks), are generally used. In the transfer mask, in general, a fine pattern formed of a metal thin film or the like is provided on a transparent glass substrate. The photolithography method is also used in manufacturing this photomask.
In recent years, the pattern of the semiconductor device has become significantly finer. In forming a finer pattern of the semiconductor device, in addition to forming a finer mask pattern in a transfer mask, there is a need to use an exposure light source having a shorter wavelength in photolithography. Specifically, in recent years, the wavelength of the exposure light source used in producing the semiconductor device has become shorter from KrF excimer laser (wavelength: 248 nm) to ArF excimer laser (wavelength: 193 nm).
Moreover, as types of the transfer mask, in addition to a related-art binary mask including a light shielding film pattern made of a chromium-based material on a transparent substrate, a halftone-type phase shift mask is known. This halftone-type phase shift mask has the structure including a semi-transparent film (phase shift film) on the transparent substrate. This semi-transparent film is configured to transmit light having an intensity that does not substantially contribute to exposure (for example, 1% to 20% with respect to an exposure wavelength), and has a predetermined phase difference. For the semi-transparent film, a material made of a transition metal silicide-based compound, for example, molybdenum silicide is widely used. This halftone-type phase shift mask includes a semi-transparent portion, which is obtained by patterning the semi-transparent film, and a light transparent portion, in which the semi-transparent film is not formed, and which transmits light having an intensity that substantially contributes to exposure, to establish a relationship in which a phase of the light that has been transmitted through the semi-transparent portion is substantially reversed with respect to a phase of the light that has been transmitted through the light transparent portion so that light beams that have passed through a vicinity of a boundary portion between the semi-transparent portion and the light transparent portion and turned into each other's region by a diffraction phenomenon cancel each other, to thereby make a light intensity in the boundary portion substantially zero and increase a contrast, that is, a resolution in the boundary portion.
Incidentally, with the reduction in wavelength of the exposure light source in recent years, deterioration of the transfer mask caused by repeated use of the transfer mask has become significant. In particular, in a case of a phase shift mask using the above-mentioned transition metal silicide-based material, due to irradiation with ArF excimer laser (wavelength: 193 nm) from the exposure light source, changes in transmittance and phase difference occur, and a phenomenon in which a line width is changed (increased) further occurs. In the case of the phase shift mask, such changes in transmittance and phase difference are serious problems that affect mask performance. When the change in transmittance is increased, transfer accuracy is reduced, and when the change in phase difference is increased, it becomes harder to obtain the phase shift effect in the pattern boundary portion, the contrast in the pattern boundary portion is reduced, and the resolution is significantly reduced. In addition, the change in line width also reduces critical dimension (CD) accuracy of the photomask, and eventually CD accuracy of a semiconductor substrate on which the transfer is performed.